Airborne gases and vapors, such as carbon monoxide and mercury, are often difficult to detect; they may be colorless and odorless or toxic at levels below which they can be seen or smelled by an average person. Hydrogen sulfide (H.sub.2 S) can be detected by the nose at the parts per billion level. However, if one has been exposed to high levels of H.sub.2 S the nose becomes desensitized to the chemical and can no longer detect it at such low levels. Also, in many environments, the smell of certain gases may be masked by other odors that are present in the air. These airborne material toxins present a growing danger to humans in automobiles, airplanes, industrial plants, mines, homes, and other environments in which humans are present for extended periods of time.
Chemical sensors, for detecting the presence of gases and vapors have been in use for many years. For example, the use of palladium and molybdenum salts for carbon monoxide detection is described in Analytical Chemistry, Vol. 19, No. 2, pages 77-81 (1974). K. Shuler and G. Schrauzer improved upon this technology by adding a third metallic salt component which produced a self-regenerating, short-lived catalyst. This catalyst, disclosed in U.S. Pat. No. 4,043,934, uses the impregnation of a carbon monoxide-sensitive catalyst solution onto powdered silica-gel substrates to produce detectors that are sensitive to low concentrations of atmospheric carbon monoxide. However, while this system is effective in detecting carbon monoxide, it has not met with commercial acceptance due to the short functional life of the sensor.
U.S. Pat. No. 5,063,164 disclosed a method to allow the carbon monoxide sensor system to be useful in consumer products, i.e. operate for at least one year without maintenance or calibration. However, the formulations disclosed in U.S. Pat. No. 5,063,164 do not pass UL 2034 standards published on Apr. 30, 1992 with respect to sensitivity and false alarm under all conditions, and specifically, under conditions of high humidity, without further modifications. The sensors disclosed in U.S. Pat. No. 5,063,164 are characterized as having a low sensitivity to carbon monoxide, a slow regeneration time, and respond in a narrow humidity range.
It is, therefore, desirable that a carbon monoxide sensor system be constructed having sensors that are formulated to provide improved sensitivity, quicker regeneration, and response in a wider humidity range than known carbon monoxide sensor systems. It is desirable that the carbon monoxide sensors meet or exceed required UL 2034 standards and have an extended functional life of at least three years normal operation.
UL Standard 2034 as published Apr. 30, 1992, contains the following criteria for a carbon monoxide sensor:
Criterion 1: Preconditioning for about 48 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and about 85.+-.5% relative humidity followed by exposure to about 200 parts per million carbon monoxide for about 35 minutes after which the ratio of light transmitted through the sensor system before the exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is equal to or greater than about 2:1;
Criterion 2: Preconditioning for about 3 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and about 15.+-.5% relative humidity followed by exposure to about 200 parts per million carbon monoxide for about 35 minutes after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is equal to or greater than about 2:1;
Criterion 3: Preconditioning for about 24 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and 53.+-.3% relative humidity followed by exposure to about 15-20 parts per million carbon monoxide for about 8 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and about 53.+-.3% relative humidity after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor immediately after exposure to carbon monoxide is equal to or less than about 1:1;
Criterion 4: Preconditioning for about 168 hours in a controlled atmosphere of about 52.degree. C. and about 95% relative humidity followed by exposure to about 400 parts per million carbon monoxide for about 15 minutes in a controlled atmosphere of about 20.degree.-25.degree. C. and about 53.+-.3% relative humidity after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is equal to or greater than about 2:1;
Criterion 5: Preconditioning for about 240 hours in a controlled atmosphere of about 61.degree. C. and about 93% relative humidity followed by about 3 hours in a controlled atmosphere of about 61.degree. C. and about 85% relative humidity followed in turn by exposure to about 400 parts per million carbon monoxide for about 15 minutes in a controlled atmosphere about 61.degree. C. and about 85% relative humidity after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is greater than or equal to about 2:1;
Criterion 6: Preconditioning for about 30 days in a controlled atmosphere of about 70.degree. C. and a low relative humidity followed by a decrease in temperature to about 49.degree. C. and a relative humidity of about 50.+-.20% relative humidity for about 1 hour followed in turn by exposure to about 200 parts per million carbon monoxide for about 35 minutes after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is equal to or greater than about 2:1;
Criterion 7: Preconditioning for about 72 hours in a controlled atmosphere of -40.degree. C. and a low relative humidity followed by an increase in the temperature to 0.degree. C. and 15.+-.5% relative humidity and exposure to 200 parts per million carbon monoxide for about 35 minutes after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is greater than or equal to about 2:1;
Criterion 8: Preconditioning for about 24 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and 53.+-.2% relative humidity followed by exposure to about 100 parts per million carbon monoxide for about 80 minutes under ambient conditions after which the ratio of light transmitted through the sensor system before exposure to carbon monoxide to light transmitted through the sensor system immediately after exposure to carbon monoxide is greater than or equal to about 2:1;
Criterion 9: Preconditioning for about 48 hours in a controlled atmosphere of about 20.degree.-25.degree. C. and about 85.+-.5% relative humidity followed by exposure to 200 parts per million carbon monoxide for about 35 minutes followed in turn by exposure for about 24 hours to an atmosphere of carbon monoxide free air after which the difference between the light transmitted through the sensor system after exposure to carbon monoxide free air and the light transmitted through the sensor system after exposure to carbon monoxide is divided by the difference between the light transmitted through the sensor system before exposure to carbon monoxide and the light transmitted through the sensor system after exposure to carbon monoxide, the quotient then being multiplied by 100 and the final result having a value greater than or equal to about 90.